Polymer film electret microphones have generated an increased interest in telephony due to their relatively high output electrical signals, their low sensitivity to external mechanical vibrations, and their immunity to electromagnetic signal interferences. An article by J. C. Baumhauer, Jr. et al. entitled "The EL2 Electret Transmitter: Analytical Modeling, Optimization, and Design" published in The Bell System Technical Journal, Vol. 58, No. 7, Sept. 1979, pages 1557-1578, discusses the basic operation of an electret transducer in general, and describes in particular an electret microphone transmitter used primarily in the Type 4A Speakerphone hands-free-answer system manufactured by Western Electric Co., Inc.
Shown in FIG. 5 of the above Baumhauer article, and further described in an article by S. P. Khanna et al. entitled "The EL2 Electret Transmitter: Technology Development" in The Bell System Technical Journal, Vol. 59, No. 5, May-June 1980, pages 745-762, the electret transmitter subassembly comprises an electret diaphragm having a gold metallization on one side thereof. A spring clip in combination with a clamping plate arrangement provides the mechanical support for the diaphragm. Moreover, the spring clip/clamping plate structure is necessary to maintain a required tension in the diaphragm. Various polymeric fluorocarbon films suitable for making electrets (such as polytetrafluoroethylene (PTFE, FEP, ETFE, CTFE) exhibit mechanical anisotropy resulting from their respective processes of manufacture. For example, when a TEFLON.RTM. FEP film is heated above approximately 100.degree. C. and cooled to room temperature, such film exhibits an elongation along its longitudinal direction and a shrinking along its transversal direction. An inherent problem with such a film is that its anisotropy at elevated temperature causes the electret film to wrinkle. Unwrinkling of the film would require heating it and applying some tension in its transversal direction. The foregoing would result in dislocations of the thin metallization layer due to the difference between its thermal expansion coefficient and that of the film.
An alternative to rectangularly shaped electret transducers is described in U. S. Pat. No. 4,249,043 to A. J. Morgan et al. wherein a circular electret foil is heated and bonded to a retaining circular ring using a cyanoacrylate adhesive. In light of the embodiments described in FIGS. 2 and 4 of this Morgan et al. patent, the thermal radial tensioning of the electret foil is not sufficient since the backplate has a protruding flange for further stretching the electret foil. Furthermore, cyanoacrylate adhesive joints between the electret foil and the ring were found unreliable when exposed to adverse environmental aging conditions of temperature and humidity (such as 85% relative humidity at 85.degree. C.). Moreover, cyanoacrylates have relatively fast curing times resulting in various storage and handling constraints in a manufacturing environment.
Therefore, there exists a need for an electret transducer/microphone exhibiting high reliability and designed to meet high volume production requirements.